Semiconductors are materials generally defined as having electrical conductivities somewhat between the high conductivity characteristics of metals, and the low conductivity characteristics of insulators. Since the invention of the transistor, the development of electrical devices based on semiconductors has revolutionized the electronics industry.
At the present time, silicon remains the most common semiconductor material for doping and device manufacture, although in recent years much interest has been generated in other semiconductor materials including gallium arsenide (GaAs) and indium phosphide (InP). Many techniques exist for producing pure crystals of these basic elements and compounds and for fabricating them into devices and circuits.
Another material toward which much interest has been directed, but for which limited success in producing practical crystals and devices has been achieved, is silicon carbide (SiC). As a semiconductor material, silicon carbide offers a number of advantageous characteristics which have long been recognized, one of which is its high thermal stability. As an example, although silicon vaporizes at temperatures of approximately 1400.degree. C., silicon carbide remains stable at temperatures approaching 2800.degree. C. Fundamentally, this means that silicon carbide can exist as a solid at extremely high temperatures at which silicon--and silicon based electronic devices--would not only be useless, but completely destroyed.
Second, silicon carbide has a relatively wide band gap, i.e. the energy difference between its valence and conduction bands, of approximately 2.2 electron volts (eV) (beta) or 2.86 C (6H - alpha). This is a rather large gap in comparison to that of silicon (1.1 eV) and means that electrons will have less tendency to move from the valence band to the conduction band solely on the basis of thermal excitation. In practical terms, this allows for silicon carbide-based devices to operate at higher temperatures than equivalent silicon-based devices.
Additionally, silicon carbide has a high thermal conductivity, a low dielectric constant, a high breakdown electric field, and a high saturated electron drift velocity, meaning that at high electric field levels devices made from it can perform at higher speeds than devices made from any of the other conventional semiconductor materials.
Because of all these inherent characteristics, silicon carbide has been a principal and perennial candidate material for application at high temperature, high power, and high speed requirements.
In order to produce a useful electronic device from a semiconductor such as silicon carbide, however, the semiconductor must have some capability for allowing the flow of conducting species from place to place. The two most common species for carrying charge are electrons and holes. Electrons are one of the fundamental subatomic particles carrying negative charge, while "holes" represent an energy level position within an atom where an electron could be placed, but form some reason is temporarily or permanently absent. Because holes are left behind when electrons move, holes can also be thought of as moving from place to place and as carrying positive charge.
In silicon carbide, both the silicon atoms and the carbon atoms have identical valence (or "outer shell") electron populations: i.e., four valence electrons. Other than crystal lattice defects and ordinary thermal population of different energy levels by the electrons, there is no encouragement for electrons or corresponding holes to move from atom to atom, and thereby carry a flow of current. If, however, an appropriate number of slightly different atoms can be added to the crystal, for example aluminum (Al), phosphorous (P), or nitrogen (N), a more conductive material will result. The greater conductivity results because atoms such as nitrogen have five valence electrons, while those such as aluminum have only three. Thus, the presence of some nitrogen atoms in a silicon carbide crystal provides a number of extra electrons which would not be present in a purer silicon carbide crystal. These extra electrons can be encouraged to move from the nitrogen atoms to empty electron positions in the silicon or carbon atoms, resulting in a flow of current. In a similar but opposite manner, the presence of atoms such as aluminum which have only three valence electrons provides available electron positions into which other electrons can move from the silicon or carbon atoms, and thereby carry current.
In terms familiar to the semiconductor industry, such added atoms are referred to as "dopants," and the process of introducing them into semiconductor materials is known as "doping." By doping a semiconductor material such as silicon carbide with either atoms with more valence electrons (n-type doping) or fewer valence electrons, (p-type doping); a semiconductor material can result which has certain specific electrical characteristics, and through which current can be made to flow under particular controlled conditions. Such materials can be fabricated into devices of many types, common examples of which are diodes, capacitors, junction transistors, and field effect transistors, all of which in turn can be built up into circuits and more complicated devices.
Accordingly, in order to produce useful semiconductor electronic devices, at least three basic requirements must be met: first, an appropriate semiconductor material must by available, often in the form of a single crystal or a monocrystalline thin film; second, the ability must exist to dope the semiconductor material in the desired manner; and third, proper techniques must be developed for forming devices from the doped materials.
Accordingly, much interest, research, publication activity, and indeed patent literature, has been directed at producing silicon carbide, doping its, and manufacturing devices from it. In spite of this high level of attention, commercial devices formed from silicon carbide have to date failed to move beyond the literature or the research lab into the commercial marketplace.
Two main categories of failure exist: a lack of any reproducible and precise methods for forming the necessary single crystals of silicon carbide essential for device manufacture; and the lack of successful doping techniques which to date have failed to result in doped monocrystalline silicon carbide having high enough purities, low enough defect levels, and sufficient electrical activation of the dopant species to form any commercially useful devices.
Recently, however, techniques have been developed for successfully growing monocrystalline silicon carbide of high purity and low defect level in each of the two most common forms of silicon carbide, the cubic or beta structure and the hexagonal 6H alpha structure. These developments are the subject of co-pending patent applications assigned to the assignee of the present invention, specifically "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 07/113,921, filed Oct. 26, 1987, now U.S. Pat. No. 4,912,063, and "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 07/113,573, filed Oct. 26, 1987, now U.S. Pat. No. 4,912,064.
From the doping standpoint, a number of methods exist for introducing an appropriate dopant into a semiconductor substrate. These include diffusion, chemical vapor deposition, and ion implantation. In ion implantation, ions of the dopant atoms are formed by any appropriate method, for example by application of a strong enough field to strip one or more electrons from each dopant atom. The ions are then accelerated, typically through a mass spectrometer to further separate and accelerate the desired dopant atoms, and finally directed into a target material (usually a single crystal or monocrystalline thin film) at energies typically between about 50 and about 300 kilo electron volts (KeV). On an atomic level, this results in severe collisions between the accelerated ions and the atoms of the target crystal. This initially can result in two problems. First, the dopant ions may not be in positions at which electrons and holes can be transferred, i.e. they are not yet "electrically activated." Secondly, a great deal of damage to the target crystal results, and in particular, atoms in the crystal lattice are dislodged from their proper positions to a greater or lesser extend. As is known to those familiar with semiconductor crystallography, such damaged crystals often do not have the electrical properties required for useful semiconductor devices. Accordingly, various attempts and techniques have been developed for dealing with the damage done during implantation.
A first technique is to heat or anneal the doped substrate material following implantation. This heating step, when followed by an appropriate rate of cooling, should theoretically encourage the atoms in the crystal lattice, most of which are atoms of the semiconductor, to recrystallize in an orderly fashion, thus repairing the damage to the semiconductor crystal lattice and allowing the dopant atoms to position themselves for electrical activation. Nevertheless, crystal defects such as dislocations, stacking faults, twins, other defects or combinations of the same, tend to remain following such annealing.
Other researchers have attempted to raise the temperature of the target during implantation, an example of which is U.S. Pat. No. 3,293,084 to McCaldin (Dec. 20, 1966), which discusses ion implantation of silicon, germanium or silicon and germanium alloys with sodium, potassium, rubidium, or cesium as the doping atom. In 1966, however, analytical tools such as transmission electron microscopy (TEM) and deep level transient spectroscopy (DLTS) were unavailable and the residual defects formed under these conditions remained undiscovered.
Accordingly, the more recent development of these analytical tools has demonstrated that ion implantation in silicon conducted at high temperatures results in lattice defects. As presently understood, when silicon is ion implanted at high temperatures, sufficient energy is imparted to the lattice by the incoming ions to cause individual point defects to arrange themselves in a lower energy configuration. These configurations include planar (stacking faults) and line (disclosure or loops) defects, with line defects forming somewhat more often. These defects are, of course, detrimental to the operation of any resulting device formed from that material.
Accordingly, in recent years, emphasis has shifted to ion implantation which is conducted when the target is at a rather low temperature, specifically temperatures on the order of the boiling point of liquid nitrogen (77.degree. K., -196.degree. C.). Under such circumstances, the implantation bombardment of ions creates a totally amorphous region in the target crystal, i.e. one in which no specific crystal structure is present. Performing annealing following the low temperature implantation encourages the implanted region--i.e. the layer represented by the depth to which the bombarding ions have penetrated--to recrystallize into a layer which resembles an epitaxial growth portion, giving this technique the name "solid-phase-epitaxy." Under these conditions, the majority of the defects formed by the initial bombardment remain at the boundary between the recrystallized layer and the layer which was too deep in the crystal to be affected by the bombardment.
Such low temperature implantation followed by annealing represents today's best technology for producing doped silicon materials for electrical devices. Indeed, the quality and performance of electrical devices formed from any given semiconductor material is one of the best indications of the quality of the original material and of the doping technique used to give it its desired properties.
Accordingly, in attempting to find a suitable ion implantation technique for adding dopants to silicon carbide, researchers have attempted to reproduce those techniques found successful with silicon alone. For example, Tohi et al., U.S. Pat. Nos. 3,629,011 and 3,829,333, discuss techniques for implanting ions in silicon carbide at room or "ordinary" temperatures or at relatively low temperatures following which the bombarded silicon carbide is annealed at high temperatures (up to 1600.degree. C.). To date, however, this has proved unsuccessful in producing any device quality doped silicon carbide crystals. In an attempt similar to those described by Tohi, Edmond et al. found that implanting dopants into silicon carbide at liquid nitrogen temperatures indeed produced amorphous layers, but annealing resulted in polycrystalline forms of silicon carbide or defective single crystals or silicon carbide, neither of which were suitable for manufacturing electrical devices, J. A. Edmond, S. P. Withrow, H. S. Kong and R. F. Davis, Beam Solid Interactions and Phase Transformations, edited by H. Kurz, G. L. Olson and J. M. Poate (Materials Research Society, Pittsburgh, 1986), p. 395.
Other attempts have likewise been made to implant silicon carbide at ordinary or "room" temperatures. At room temperatures (approximately 293.degree. K. to 298.degree. K.), no technique which produces consistent results has been developed. Some crystals tend to remain crystalline, while others become amorphous in a manner similar to that which occurs that upon low temperature implantation. The layers which remain crystalline during bombardment tend to recover properly upon annealing, but certain problems in the technique remain. At certain dosages of the dopants (dosage control being a requirement for imparting desired electrical properties to the target material), the crystal structure became amorphous. Furthermore, successful annealing has to be conducted at temperatures of approximately 1800.degree. C. These temperatures are wall above the vaporization point of the silicon substrates upon which silicon carbide was always deposited prior to the concurrent inventions discussed earlier which produce silicon carbide upon silicon carbide.
Accordingly, there are no known successful techniques in the art for producing device quality, single crystal, electrically activated doped silicon carbide semiconductive materials using conventional ion implantation techniques.
It is therefore an object of the present invention to provide a method of producing either n or p-doped regions in silicon carbide suitable for semiconductor electrical devices.
It is another object of the invention to produce appropriately doped monocrystalline silicon carbide in a manner which prevents amorphization of the silicon carbide crystal during the doping technique while appropriately electrically activating the dopant introduced.
It is a further object of the invention to provide a method for producing doped monocrystalline silicon carbide by ion beam implantation.
It is another object of the invention to provide a method for producing a doped silicon carbide by ion beam implantation conducted at temperatures high enough to prevent amorphization of the silicon carbide followed by removal of the highly conductive near surface layer typically formed from ion implantation, and by electrical activation of the dopant atoms introduced by the ion beam.
It is a further object of the invention to produce electrical devices of useful commercial quality using doped portions of silicon carbide formed according to the ion implantation techniques of the present invention.